Sharks, like all fish, breathe through gills. Gills are respiratory organs performing the same function as lungs in land animals. A shark's gills contain hundreds of feathery filaments with thousands of lamellae per filament. Lamellae contain tiny blood vessels or capillaries. Capillaries absorb oxygen from water and release carbon dioxide. Human lungs contain capillaries for the same purpose. Sharks absorb 80% of the 1% oxygen concentration in water. Human lungs absorb a meager 25% of the 21% oxygen concentration in air.

Sharks have 5 to 7 gill arches with a single gill slit in each arch. Most fish have an operculum, or covering, over their gills. Sharks do not have an operculum, but they do have structural gill rakers perpendicular to the gill arches. Sharks breathe through ram ventilation. Ram ventilation occurs as water flows over the shark's gills. Water passes through the mouth and over gills as a shark swims forward. Sharks have an additional gill slit, or spiracle, directly behind their eyes. The spiracle directs water to the mouth, and gill rakers direct water over the gill slits.

Sharks have hearts with two chambers. Oxygenated blood travels from the gills to the atrium of the shark's heart. The heart pumps oxygenated blood into arteries to reach tissue and organs throughout the body. Deoxygenated blood travels through veins to the ventricle of the heart. The ventricle pumps deoxygenated blood to the gill capillaries. A shark's heart is not very strong - the flow of water helps move blood through the shark's circulatory system, so the heart does not provide all of the necessary force to keep blood flowing.

The entire process of ram ventilation and circulation in a shark becomes more efficient as the shark swims faster. The accelerated process during faster swimming is called the ramjet principle. The ramjet principle helps sharks chase prey without tiring. A shark's blood circulates in a counter-current flow that always moves in the opposite direction of the water, which helps to maximize oxygen absorption from the sea.

Sharks are often referred to as living fossils because current species were found far back in the fossil record. Despite the long lineage of modern-day shark species, ancient species existed before them. The oldest sharks used buccal pumping to breathe when they were not moving. Buccal pumping uses the buccal, or cheek, muscles to pull water into the mouth and over the gills. Bony fish and some modern-day shark species, such as nurse sharks, angel sharks, and carpet sharks, still use buccal breathing.

Most sharks able to use buccal pumping are bottom-feeders. They frequently rest on the ocean floor and have a dorsoventrally flattened body. Their bodies are flat and thin along their backs. Buccal breathing sharks have prominent spiracles behind the eyes. The spiracles pull water in and push it back out when the shark is buried in the sand on the ocean floor.

Many sharks gradually lost physical structures necessary for buccal pumping as they evolved. The spiracles of large, fast sharks shrank and disappeared as they evolved into sleek, modern predators. Ram ventilation is more energy-efficient than buccal breathing. The water flow as a shark swims uses less energy than pumping water into the mouth. Most modern shark species can alternate between buccal pumping and ram ventilation. The sand tiger shark alternates between breathing methods frequently because it hunts prey on the ocean floor and swimming through water.

Some sharks are obligate ram ventilators. They have completely lost the ability to breathe through buccal pumping. Approximately 24 of 400 known shark species are obligate ram ventilators. These sharks swim continuously and drown if they can not swim. Great whites, mako, salmon sharks, and the whale shark only breathe through ram ventilation. Their spiracles are barely visible, and the great white does not have spiracles anymore.

Scientists believe sharks that breathe only through obligate ram ventilation use features of the water to rest without drowning. Temperature, salinity, and time of day affect the oxygen concentration of ocean water. Scientists discovered motionless reef sharks in the Caves of the Sleeping Sharks during the 1970s. Reef sharks breathe with obligate ram ventilation. The scientists theorized that the exceptionally high concentration of oxygen in the cave waters allowed the reef sharks to remain still.